In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent properties with respect to heat resistance, impact resistance and transparency. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound, such as 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as “bisphenol A”), and phosgene has been commercially practiced.
However, the interfacial polycondensation process has problems in that it is necessary to use phosgene, which is poisonous, that a reaction apparatus is likely to be corroded with chlorine-containing compounds, such as hydrogen chloride and sodium chloride, which are by-produced, and methylene chloride which is used as a solvent in a large quantity, and that difficulties are encountered in separating and removing impurities (such as sodium chloride) and residual methylene chloride, which adversely affect properties of a produced polymer. As a method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, a melt transesterification process has conventionally been known, in which an aromatic polycarbonate is produced by performing an ester exchange reaction between an aromatic dihydroxy compound (such as bisphenol A) and a diaryl carbonate (such as diphenyl carbonate) in the molten state. Contrary to the interfacial polycondensation process, the melt transesterification process has an advantage in that a solvent need not be used. However, in the melt transesterification process, the reaction must be performed under high temperature and high vacuum conditions, thus posing a problem in that the produced aromatic polycarbonate is likely to suffer a discoloration. In addition, since the viscosity of a molten polycarbonate is high, it is difficult to remove foreign matter, especially foreign matter particles having an extremely small size, from the polycarbonate. The foreign matter particles having an extremely small size are causatives of optical defects and, when the obtained aromatic polycarbonate is used in the optical application field, especially in the production of an optical disk, the presence of extremely small foreign matter particles contained in the polycarbonate causes a bit-error. To solve this problem, there has been proposed a method in which the foreign matter materials are removed from low viscosity materials, such as raw materials or a reaction mixture in the early stage of the polymerization reaction.
In this situation, the present inventors proposed a method for producing a colorless aromatic polycarbonate by a melt transesterification reaction in a stainless steel reactor, wherein the stainless steel reactor is washed with a liquid containing an aromatic hydroxyl compound (see, for example, Patent Document 1). By the use of this method, it became possible to produce an aromatic polycarbonate having highly improved colorlessness. However, from the later studies, it became apparent that this method has the following problems. With respect to the aromatic polycarbonate obtained at the beginning of the production process which is started after the washing of the reactor, the terminal hydroxyl group ratio of the produced aromatic polycarbonate is likely to become varied and, hence, a production loss is likely to occur.
As another proposal for reducing the amount of foreign matter contained in an aromatic polycarbonate, there can be mentioned a method in which an aromatic polycarbonate is produced using a reactor-pipeline system containing at least two reactors, wherein a filter is secured in a pipe of the reactor-pipeline system at a position upstream of a final reactor (see, for example, Patent Document 2). Further, there can be mentioned a method in which the above-mentioned reactor-pipeline system is provided with a filter switching device for switching between filters for alternative use in the reactor-pipeline system (see, for example, Patent Document 3). In this method, before performing a switching from a used filter to a new filter which are set in the filter switching device of the reactor-pipeline system, the new filter is subjected to washing with a washing agent which is an aromatic hydroxyl compound containing a basic compound, followed by discharge of the used washing agent to the outside of the reactor-pipeline system. After the discharge of the used washing agent, the used filter is switched to the washed new filter by the filter switching device. The amount of extremely small foreign matter particles contained in the produced aromatic polycarbonate was considerably decreased by this method. However, in this method, it is necessary that a new filter set in the filter switching device be washed for a long period of time. Further, with respect to the aromatic polycarbonate obtained at the beginning of the production process which is started after the washing of the filter, the terminal hydroxyl group ratio is likely to be varied and, hence, a production loss is likely to occur.    Patent Document 1: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-56984    Patent Document 2: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-234845    Patent Document 3: Unexamined Japanese Patent Application Laid-Open Specification No. 2003-34722